The invention deals generally with joints and more specifically with the bonding of materials of diverse thermal expansions by the use of an intermediate additional material.
The problem of joining materials with substantial differences in thermal expansion has existed for a long time. The difficulties with such bonds became obvious early in the development of electronic tubes because of the necessity of bringing electrical connections through the glass envelope of a tube, and they continue today even in semiconductor technology, because in that field there is also a requirement for bonding electrical conductors to electrical insulators.
However, the problem of such bonds is most severe in applications where significant heat or power is involved. Under circumstances where materials are raised to high temperatures, the differences in both the coefficient of thermal expansion and the actual dimensions of the materials are greatly increased. It is such exaggerated differences which usually cause the failure of bonds between materials. Ironically, material research technology which has dramatically increased the availability of new high temperature materials has only made the problem of differences in thermal expansion more severe. For instance, the new very high temperature material identified as carbon/carbon has a thermal expansion of essentially zero, and that means that there are difficulties in bonding it to virtually every metal, because metals all have significant coefficients of thermal expansion.
The classic manner of bonding two materials with different rates of thermal expansion has been to use a bonding material between them which has a coefficient of thermal expansion which is intermediate to the coefficients of thermal expansion of the materials being bonded. In that way, the stress of the differences in expansion of the materials is divided between the bonds on the two sides of the intermediate material. In fact, in severe cases of either large differences in thermal expansion coefficients or exceptionally fragile materials, several intermediate materials, each with progressively different thermal expansion coefficients have been used. This technique divides the thermal stress among more surface bonds and therefore decreases the likelihood of failure.
Another method of accommodating structures to the differences in thermal expansion of the materials used in their construction is to design the parts with mechanical strain relief. The simplest example of such a structure might be a very thin metal cylinder bonded to the end of a ceramic cylinder, a structure which appears frequently in high power electron tubes. In such a structure, the flexibility of the thin metal cylinder actually permits the cylinder to distort as it expands, and thereby limits the stress on the joint.
Both of these techniques of the prior art are difficult and expensive to implement. Moreover, since one requires the selection of specific intermediate materials and the other requires special structures, they are not always available for all applications, regardless of cost.